Flood Risk Management using HEC-RAS Rain On-Grid Model for the 2024 March – April - May El-Nino Flood Event: A Case Study of Nyando River Basin, Kenya

Abstract

Flood events have led to several casualties and widespread suffering among populations worldwide. The Nyando River Basin has historically been susceptible to perennial floods during El Niño flood events. This study, therefore, aimed to assess flood risk in the Nyando River Basin, Kenya, for the 2024 March – April - May El Niño flood event, using an integrated hydraulic and hydrological approach with the HEC-RAS Rain-on-Grid model. The research specifically assessed the quality of rainfall products for flood modelling, simulated the 2024 MAM El Niño flood event, calibrated it with satellite data, citizen science contributions and ground observations and assessed the effectiveness of nature-based solutions measures within the Nyando River Basin. The Pearson correlation coefficient (R²) for the GPM IMERG-Late run V07 and CHIRPS was determined. The HEC-RAS Rain-On-Grid model was configured to simulate the 2024 MAM El Niño flood event. Finally, Nature-Based Solutions were simulated within the HEC-RAS Rain-on-Grid model. Findings showed that the adjusted GPM IMERG Late Run V07 satellite precipitation product produced R² values of 0.7288 at the Kericho station and 0.5568 at the Kisumu station. The corrected CHIRPS yielded R² values of 0.6411 at Kericho and 0.6245 at Kisumu. For modelling the flow during the MAM 2024 El Niño flood event, an initial abstraction ratio of 0.05, the minimum Manning’s N value, and a 5% increase in curve numbers for CN < 95 and 2% for CN ≥ 95 were adopted, resulting in a peak discharge of 503.8 m³/s. The scenario with maximum infiltration, representing well-vegetated land with primarily HSG A and B soils, demonstrated the lowest peak discharge of 238 m³/s and a reduced runoff volume of 85 m³. The study found that the most effective flood risk management strategy involves implementing nature-based solutions such as increasing forest cover, effectively lowering the peak discharge and runoff volume in the Nyando River basin.